1. Field of the Invention
The present invention concerns a device for access to and follower control of a track borne by a medium, such as an optical disk, of information that is optically readable and/or recordable by focused radiation.
2. Description of the Prior Art
For in the case of the writing and/or the reading of an optical disk, it is necessary to gain access to a zone of the disk as well as to thereafter keep to this zone with high precision, both radially and perpendicularly to the surface of the disk. Thus, these prior art devices generally have three separate motor devices.
Now, for fulfilling all the above three functions, there already has been a description given in the French patent No. 83 03425 corresponding to U.S. Pat. No. 4,554,643, of an access and follower control device using only two motor devices. In this case, the access coil is used to fulfil both the access and the radial tracking functions. However, to obtain efficient operation, there is provision for flexible connection means between the coil and the sliding elements of the coil that provide access and radial tracking in the gap of a fixed magnetic circuit. For, the use of means in a simple sliding operation does not make it provide for satisfactory precision in the tracking of the track. Furthermore, in the French patent No. 82 03425 corresponding to U.S. Pat. No. 4,554,653, to prevent the focusing operation from being accompanied by a diaphragm effect on the optical system, the coils of the two motor devices are connected to each other by means of a flexible attachment, each coil being fixedly joined to an element of the optical head proper. The structure of the access and follower control device thus made increases the unbalance of the movable element and hence the mass of the balancing counterweight, the chief drawback of this being that it increases the weight of the head.
In pursuing studies on the access and follower control devices for an optical disk, the Applicant has perceived that, by using a single-piece structure for the assembly formed by the optical head and the two coils provided for in the motor devices, it is possible, as shall be explained further below, to get rid of the flexible connection means while at the same time preserving excellent precision of the positioning and thus lightening the moving element. This makes it possible to accelerate the access to the track.